An outboard motor mounted in the rear part of a hull creates thrust in the hull by causing a propeller to rotate using the motive power of an engine. Exhaust created by the driving of the engine is emitted into the water via an exhaust passage. The outboard motor further comprises a shift mechanism for shifting the traveling direction of the hull between forward and reverse. By operating the shift mechanism and shifting the engaged state of the clutch, the rotating direction of the propeller can be reversed. In other words, the clutch shifts the rotating direction of the propeller to forward or reverse in relation to the rotating direction of the engine.
At a point in time when the shift mechanism switches from forward to reverse while the hull is traveling, the flow of water created by the propeller continues in the direction of propelling the hull forward. Therefore, a phenomenon of so-called drag-induced counter-rotation can occur in which the propeller continues to be caused to rotate in the forward-moving direction (the forward rotation direction) by the flow of water in the forward-moving direction. However, the rotating direction of the propeller has been shifted from forward to reverse by the clutch. The engine might be rotating in reverse depending on the operating state. Particularly, the engine could begin to rotate in reverse when the shift mechanism is shifted to reverse while the boat is traveling at a high speed. When the engine rotates in reverse, negative pressure is created in the exhaust passage. As a result, there is a possibility of water being drawn into the exhaust port of the engine via the exhaust passage. Such water intake is preferably eliminated.
For example, Japanese Patent Application Laid-Open Publication No. 2002-349257 (JP 2002-349257 A) discloses an outboard motor designed to prevent water from being drawn in. This outboard motor has a structure in which the exhaust passage of the exhaust system of the engine is communicated with an air intake box of an air intake system of the engine via a communication passage and a one-way valve. The one-way valve is configured so that air is drawn in only to the exhaust passage from the air intake box. When negative pressure is created in the exhaust passage by the reverse rotation of the engine, atmospheric air flows into the exhaust passage via the communication passage and the one-way valve. As a result, negative pressure states in the exhaust passage are eliminated, and water is therefore prevented from entering the exhaust passage. Negative pressure states in the exhaust passage are preferably eliminated more quickly in order to effectively prevent water from entering the exhaust passage.